Suppose you are in a busy environment and your device is lying around somewhere. You sometimes use it but cannot look after it all the time. A sort of high risk environment for phone theft where you could put it in anti theft mode which is goes into an alarm mode when a custom user selection of the following occurs:

it will display a warning on a red background flashing in black: You are committing a crime. Put this phone back, now! (or similar). Make sure is it a positive stimulus for the perpetrator.

do nothing. It may simulate a fake network error or similar when a number is dialed. It may provide limited functionality so that the device is not thrown away as trash or users data is being accessed or high costs are being generated.

If it is not unlocked after predefined number of seconds may:

set off a loud alarm

start sending messages to a predefined email address with live data like pictures from the camera, audio recordings from the microphone, GPS coordinates, logs of numbers dailed, URLs surfed to and messages send, WiFi network and USB ID's for retracing the location of the device and identifying the perpetrator. (Compress the attachments of the email.)

activate self-destruct mechanism

If the above anti-theft measures fail

a special text message could be sent to the phone which erases all contacts then activates the pin lock for the phone and SIM card.

activation of some sort remote control

In addition.
If you've put it down and forgotten about it, somebody picks it up, it would first ask for the PIN, with a message on-screen that says something like "If you don't know PIN, give 1234".

If user selects "Use phone", then it would present factory looking interface with empty of fake address book, etc. and quietly contact owner anyway and tell current coordinates ;-) Also logging of text input will be activated and emailed to the owner.

The idea is keep the thief using the phone until the police arrive and recover it.
If phone notices new SIM, it would go directly to "I'm stolen" mode. Maybe run under Linux-VServer http://www.linux-vserver.org/ in stolen mode.

Perhaps, assuming the phone was locked in such a manner and later found or recovered by police, the phone could be returned to "normal mode" by entering the pin AND being within 100 meters of a "home" location, defined by the GPS when you first set it up.

With suitable arrangement remote ssh connection to phone should be
possible which would give nice control over stolen phone.

There could also be a bluetooth headset and/or bluetooth watch. If it loses contact to phone, phone would complain loudly and if bluetooth gadget is enough smart it could complain too.
This gadget could warn about low battery too.

How about using Adeona, an "open source system for tracking the location of your lost or stolen laptop that does not rely on a proprietary, central service."
It'll surely need to be cutomized to require as less battery as possible, but the implementation is here, and seems to be pretty well though.

Suppose you are in a busy environment and your device is lying around somewhere. You sometimes use it but cannot look after it all the time. A sort of high risk environment for phone theft where you could put it in anti theft mode which is goes into an alarm mode when a custom user selection of the following occurs:

it will display a warning on a red background flashing in black: You are committing a crime. Put this phone back, now! (or similar). Make sure is it a positive stimulus for the perpetrator.

do nothing. It may simulate a fake network error or similar when a number is dialed. It may provide limited functionality so that the device is not thrown away as trash or users data is being accessed or high costs are being generated.

If it is not unlocked after predefined number of seconds may:

set off a loud alarm

start sending messages to a predefined email address with live data like pictures from the camera, audio recordings from the microphone, GPS coordinates, logs of numbers dailed, URLs surfed to and messages send, WiFi network and USB ID's for retracing the location of the device and identifying the perpetrator. (Compress the attachments of the email.)

activate self-destruct mechanism

If the above anti-theft measures fail

a special text message could be sent to the phone which erases all contacts then activates the pin lock for the phone and SIM card.

activation of some sort remote control

In addition.
If you've put it down and forgotten about it, somebody picks it up, it would first ask for the PIN, with a message on-screen that says something like "If you don't know PIN, give 1234".

If user selects "Use phone", then it would present factory looking interface with empty of fake address book, etc. and quietly contact owner anyway and tell current coordinates ;-) Also logging of text input will be activated and emailed to the owner.

The idea is keep the thief using the phone until the police arrive and recover it.
If phone notices new SIM, it would go directly to "I'm stolen" mode. Maybe run under Linux-VServer http://www.linux-vserver.org/ in stolen mode.

Perhaps, assuming the phone was locked in such a manner and later found or recovered by police, the phone could be returned to "normal mode" by entering the pin AND being within 100 meters of a "home" location, defined by the GPS when you first set it up.

With suitable arrangement remote ssh connection to phone should be
possible which would give nice control over stolen phone.

There could also be a bluetooth headset and/or bluetooth watch. If it loses contact to phone, phone would complain loudly and if bluetooth gadget is enough smart it could complain too.
This gadget could warn about low battery too.

How about using Adeona, an "open source system for tracking the location of your lost or stolen laptop that does not rely on a proprietary, central service."
It'll surely need to be cutomized to require as less battery as possible, but the implementation is here, and seems to be pretty well though.